It is standard practice in adaptive rate communication systems, e.g., IEEE 802.11 systems, to monitor some performance metric, for example, the received SNR (Signal-to-Noise Ratio), BER (Bit Error Rate), PER (Packet Error Rate) or similar performance metrics. Based on these measured results, the system adjusts symbol rates to provide some minimum acceptable level of performance. The symbol rates typically have rational relationships to each other to minimize system complexity. In some systems using rate adaptation, rate transitions are performed in abrupt steps by changing modulation types or modes.
Some current systems monitor a performance metric and compare a derivative form of the metric to an upper or lower threshold. If the derivative rises above an upper threshold, information throughput is raised either by increasing the symbol rate of the system, changing the modulation technique or mode, or a combination of the two. If the derivative degrades to below a lower threshold, however, information throughput is lowered by decreasing the symbol rate of the system, changing the modulation technique or mode, or a combination of the two. These current techniques require instantiations of multiple modulators and demodulators in a single modem and force discontinuous information rates as a function of error metric values.
Adaptive rate modems are commonly used in the industry. An example of an adaptive rate modem is disclosed in U.S. Pat. No. 5,541,955 to Jacobsmeyer. This modem uses soft decision metrics to provide an estimate of the signal-to-noise ratio and predicts a future signal-to-noise ratio to determine the desired data rate for the modem.
It is possible to use a pilot carrier in similar systems. For example, baseband injected pilot carriers are often used in communication systems to enhance the performance of required operations in a high performance modem. An example of such system is disclosed in commonly assigned U.S. Pat. No. 6,606,357, the disclosure which is hereby incorporated by reference in its entirety. The '357 patent discloses a QPSK modulation scheme that uses a data spreading mechanism to take a relatively limited portion of the available transmitter power and inject into the QPSK waveform a prescribed amount of carrier signal power. Detection and non-regenerative extraction of the carrier at the receiver can be achieved without incurring a signal-to-noise degradation penalty. This injected, carrier based modulation scheme can use high performance, forward error correction (FEC) coding to reduce the signal power required for achieving a low energy per bit-to-noise density ratio (Eb/No) on the order of 1 to about 0 dB. It is possible that performance enhancements could be applied to this baseband injected pilot carrier system using adaptive techniques.